The present invention relates to a plant control that is capable of efficiently suppressing that a control output overshoots a desired value and that the control output lags behind the desired value.
Conventionally, there have been proposed several techniques for controlling a modeled object (such an object is also referred to as a plant). An example is disclosed in Japanese Patent Application Publication No. 2005-023922. Through use of a response assignment control, an output of a plant (that is, a control output) can converge to a desired value at a desired speed. In particular, according to a 2-degree-of-freedom response assignment control, both of a speed with which the control output follows a desired value and a speed with which an error between the control output and the desired value converges to zero when disturbance is applied to the plant can be separately specified.
When a plant has nonlinear characteristics, the control output may overshoot a desired value and/or the control output may lag behind the desired value even though the response assignment control is used. Such overshooting and delay occur, for example, due to a mechanical friction or a nonlinear element such as a spring that acts in one direction.
FIG. 19 is an example of such overshooting and delay when a lift amount of an intake or exhaust valve of an internal-combustion engine is controlled. Referring to FIG. 19(a), a desired value Lift_cmd_f (which is, more specifically, a filtered desired value Lift_cmd_f as described later) and an actually-detected lift amount Lift when the valve is opened at time t1 are shown. It is seen that there is a delay with which the lift amount Lift follows the desired value Lift_cmd_f. Referring to FIG. 19(b), the desired value Lift_cmd_f and the actually-detected lift amount Lift when the valve is closed at time t2 are shown. It is seen that the lift amount Lift overshoots the desired value. Such delay and overshooting may be caused by an actuator (a plant) that drives the valve. For example, the actuator is provided with a return spring that is biased in a direction of a closed position of the valve. When the valve is closed, the restoring force of the spring may cause the control output (lift amount) to overshoot a desired value. When the valve is opened, a delay may occur before the control output reaches the desired value because the valve is moved against the biasing force of the spring.
Such overshooting and delay may reduce the emission, fuel efficiency and drivability. Furthermore, such overshooting may damage the actuator.
However, if overshooting is suppressed too much, a delay with which the control output follows a desired value may increase. If a delay with which the control output follows a desired value is suppressed too much, overshooting may increase.
Thus, there is a need for a control that is capable of improving robustness against a plant that exhibits nonlinear characteristics while efficiently suppressing overshooting and delay of the control output relative to a desired value.